ABSTRACT

The Taylor Sandstone in Terryville field, Lincoln Parish, Louisiana is one of many tight-gas sands of the Upper Jurassic Schuler Formation, Cotton Valley Group. This coastal strandplain sandstone is fine to very fine-grained, well sorted, with grains that are subrounded to well rounded. It is highly quartzose with a great abundance of carbonate cement and lesser concentrations of quartz cement, clay precipitates, and pyrite.

Mechanical compaction has caused a ten percent reduction in primary porosity with increasing overburden. Chemical diagenesis has altered the texture and composition of the sandstone and has affected porosity and permeability through cementation, dissolution, and authigenic clay precipitation. Early stages of diagenesis included pyritization, mechanical compaction, and hinderence of compaction by precipitation of quartz cement at grain contacts. The middle stages were dominated by carbonate cementation, which replaced some detritus and quartz cement and reduced primary porosity to irreducible limits. Finally, the latest stages of diagenesis included development of secondary porosity by localized dissolution of replacive and interstitial carbonate and reduction of porosity by precipitation of pore-lining and pore-filling illite and illite/smectite clays. Poor permeabilities in the sandstone are a direct consequence of incomplete dissolution of carbonate in pore-throats and the obstruction of voids by the late clay precipitates.

The conductive property of authigenic pyrite has affected the response of the Deep Induction resistivity log, consequently causing abnormally high calculated water-saturation values in the Taylor Sandstone. The effect pyrite has on formation resistivity can be clearly seen on R_t- plots (Pickett Method). Formation resisitivity is affected even at low percentages of pyrite, and shows an exponential decrease with increasing pyrite concentration. A correction of resistivity was made possible by determining pyrite concentrations in the laboratory. Furthermore, the R- plot may confirm the presence of pyrite in a sandstone where core samples are not available.